1. Field of the Invention
The invention relates to a milling tool which is capable of simultaneously accomplishing rough milling and finish milling when a complex material comprised of a harder material and a softer material is milled.
2. Description of the Related Art
As a milling tool for milling a surface of cast alloy or ceramics, conventionally a tool was used that included an abrasive grain layer comprised of hard abrasive grains such as diamond. In such a surface milling process a, a high milling efficiency is required with a preferable finished surface roughness presenting a small number of scratches.
On the other hand, for instance, when a complex material comprised of iron and aluminum (hereinafter, referred to simply as “iron-aluminum complex material”) to be used for automobile parts such as an engine and a gear is milled, it is sometimes necessary to carry out both rough milling and finish milling to accomplish the desired dimensions of a final product. Japanese Patent Application Publications Nos. 2002-263937 and 2007-152516 have suggested a milling tool that is capable of carrying out both rough milling and finish milling by use of a single tool.
The milling tool suggested in Japanese Patent Application Publication No. 2002-263937 includes a cup-shaped base. Diamond abrasive grains are brazed on an end surface and a peripheral surface of the base to define a milling tool. A recess groove is formed at the end surface along a boundary between an outer area and an inner area. The recess groove prevents big chips generated when the outer area is roughly milled, from entering the inner area acting as a finish-milling tool, to thereby enhance a finish accuracy with which finish milling is carried out on a workpiece. Furthermore, the suggested milling tool can accomplish both rough milling and finish milling at a time.
A planar surface of the inner area is designed higher than a planar surface of the outer area, ensuring that abrasive grains arranged in the inner area can sufficiently bite a surface of the workpiece after the surface has been milled by means of the outer area. As a result, it is possible to efficiently mill the workpiece.
The milling tool suggested in Japanese Patent Application Publication No. 2007-152516 is comprised of a grinding stone including a cup-shaped base. Diamond abrasive grains are brazed onto an end surface of the base to define an abrasive layer. A recess groove is formed at the end surface along a boundary between an outer area disposed closer to the end surface and an inner area disposed more distant from the end surface. The inner area has a planar surface higher than a planar surface of the outer area. The outer area has an inclined or rounded surface at an outer edge thereof. In a portion of the inner area close to the recess groove, an inclined surface is formed downwardly inclining towards the recess groove. The portion of the inner area and a portion of the outer area close to the recess groove are almost equal in height to each other.
Particularly high loads act on abrasive grains fixed in the inner area. The milling tool having the above-mentioned structure can disperse the loads to thereby allow the abrasive grains to wear uniformly, ensuring enhancement in both accuracy with which the milling tool mills a workpiece, and the lifespan of the milling tool.
When a workpiece composed of a material having a low hardness is milled by means of a milling tool including abrasive grains fixedly brazed to a base, the abrasive grains are often blocked by the chips chips generated during milling.
In order to prevent abrasive grains from being blocked by the chips, Japanese Patent Application Publication No. 2005-279851 has suggested a rotary disc cutter including a plurality of abrasive-grain clusters spaced away from one another by a predetermined distance, each of the abrasive-grain clusters being comprised of a centrally disposed single abrasive grain, and a plurality of abrasive grains disposed on a concentric circle around the central abrasive grain.
The above-mentioned iron-aluminum complex material is accompanied with a problem that since the material has different hardness, chips generated when a harder material (that is, iron) was milled flaw a softer material (that is, aluminum), resulting in reducing an accuracy with which a workpiece is milled.
The suggested milling tool is accompanied further by a problem in that free abrasive grain(s) having fell off a base during milling a workpiece flaws both a harder material (that is, iron) and a softer material (that is, aluminum).
In the milling tool suggested in Japanese Patent Application Publication No. 2002-263937, abrasive grains disposed in the inner area can bite a workpiece well by designing a planar surface of the inner area to be higher than a planar surface of the outer area. However, since high loads act on abrasive grains disposed in the inner area in the vicinity of the recess groove, the abrasive grains are worn out irregularly, resulting in reducing both accuracy with which a workpiece is milled and a lifespan of the grinding stone.
Furthermore, abrasive grains are disposed one by one with a predetermined space between adjacent ones in the suggested milling tool. Hence, a high pressure acts on each of the abrasive grains while a workpiece is being milled, some abrasive grains are likely to fall out of the milling tool.
In addition, since abrasive grains deeply bite a workpiece, inevitably big chips are generated, causing chips resulted from a harder material to flaw a portion of the workpiece composed of a softer material.
In accordance with the milling tool suggested in Japanese Patent Application Publication No. 2007-152516, since loads are dispersed to abrasive grains disposed in the inner area of an end surface, it would be possible to allow for the uniform wear of the abrasive grains. However, since abrasive grains are disposed one by one at an edge of the end surface, the problem that abrasive grains disposed at an edge on which a highest load acts are likely to fall is not yet solved.
In the rotary disc cutter suggested in Japanese Patent Application Publication No. 2005-279851, the abrasive-grain clusters are disposed on a side surface of the rotary disc cutter. Each of the abrasive-grain clusters is comprised of a single abrasive grain, and a plurality of abrasive grains on a concentric circle around the single abrasive grain. In the rotary disc cutter, loads acting on one of the abrasive grains defining an abrasive-grain cluster while a workpiece is being milled are dispersed to the adjacent abrasive grains. Accordingly, since a force acting on each of the abrasive grains is smaller than a force acting on a singly disposed abrasive grain, the abrasive grains in the rotary disc cutter are not likely to fall off the milling center.
The inventors of the present invention tested milling workpieces composed of an iron-aluminum complex material through the use of a milling tool including the same abrasive-grain clusters suggested in Japanese Patent Application Publication No. 2005-279851, resulting in failure in obtaining an adequate milling accuracy. In particular, it was found that a centrally disposed abrasive grain in an abrasive-grain cluster has fell at an outermost area of the milling tool.
It is considered that this is because a load acting on an outermost area of the milling tool while a workpiece is being milled is much greater than a load acting on an edge of the rotary disc cutter suggested in Japanese Patent Application Publication No. 2005-279851 while a workpiece is being milled, and hence, a centrally disposed abrasive grain, that is, an abrasive grain brazed to a base with lowest bonding force among abrasive grains defining an abrasive-grain cluster falls.